/*
 *    Copyright 2009-2021 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.parsing;

import java.util.Properties;

/**
 * @author Clinton Begin
 * @author Kazuki Shimizu
 */
public class PropertyParser {

  private static final String KEY_PREFIX = "org.apache.ibatis.parsing.PropertyParser.";
  /**
   * The special property key that indicate whether enable a default value on placeholder.
   * <p>
   *   The default value is {@code false} (indicate disable a default value on placeholder)
   *   If you specify the {@code true}, you can specify key and default value on placeholder (e.g. {@code ${db.username:postgres}}).
   * </p>
   * @since 3.4.2
   */
  public static final String KEY_ENABLE_DEFAULT_VALUE = KEY_PREFIX + "enable-default-value";

  /**
   * The special property key that specify a separator for key and default value on placeholder.
   * <p>
   *   The default separator is {@code ":"}.
   * </p>
   * @since 3.4.2
   */
  public static final String KEY_DEFAULT_VALUE_SEPARATOR = KEY_PREFIX + "default-value-separator";

  private static final String ENABLE_DEFAULT_VALUE = "false";
  private static final String DEFAULT_VALUE_SEPARATOR = ":";

  /**
   * 修饰符为 private ，禁止构造 PropertyParser 对象，因为它是一个静态方法的工具类。
   */
  private PropertyParser() {
    // Prevent Instantiation
  }

  /**
   * 基于 variables 变量，替换 string 字符串中的动态属性，并返回结果。
   * @param string 替换的字符
   * @param variables Properties 对象
   * @return
   */
  public static String parse(String string, Properties variables) {
    // 创建 VariableTokenHandler 对象
    VariableTokenHandler handler = new VariableTokenHandler(variables);
    // 创建 GenericTokenParser 对象
    // openToken = ${ ，closeToken = }，就是转义之前的特殊标
    GenericTokenParser parser = new GenericTokenParser("${", "}", handler);
    // 调用 GenericTokenParser#parse(String text) 方法，执行解析。
    return parser.parse(string);
  }

  /**
   * PropertyParser 的内部静态类
   * Mybatis变量 Token 处理器
   */
  private static class VariableTokenHandler implements TokenHandler {
    /**
     * 变量 Properties 对象
     */
    private final Properties variables;
    /**
     * 是否开启默认值功能。默认为 {@link #ENABLE_DEFAULT_VALUE} (false)
     */
    private final boolean enableDefaultValue;
    /**
     * 默认值的分隔符。默认为 {@link #KEY_DEFAULT_VALUE_SEPARATOR} ，即 ":" 。
     */
    private final String defaultValueSeparator;

    private VariableTokenHandler(Properties variables) {
      this.variables = variables;
      this.enableDefaultValue = Boolean.parseBoolean(getPropertyValue(KEY_ENABLE_DEFAULT_VALUE, ENABLE_DEFAULT_VALUE));
      this.defaultValueSeparator = getPropertyValue(KEY_DEFAULT_VALUE_SEPARATOR, DEFAULT_VALUE_SEPARATOR);
    }

    /**
     * 获取property的value
     * @param key property的key
     * @param defaultValue 默认值
     * @return
     */
    private String getPropertyValue(String key, String defaultValue) {
      // 如果variables为空表示没有配置Properties 则使用默认值，否则从Properties中读取
      return (variables == null) ? defaultValue : variables.getProperty(key, defaultValue);
    }

    @Override
    public String handleToken(String content) {
      if (variables != null) {
        String key = content;
        // 开启默认值功能
        if (enableDefaultValue) {
          // 获取分隔符是否存在
          final int separatorIndex = content.indexOf(defaultValueSeparator);
          String defaultValue = null;
          //分隔符如果存在
          if (separatorIndex >= 0) {
            //根据分隔符位置获取分隔符之前的字符（获得key）
            key = content.substring(0, separatorIndex);
            //获取分割符之后的字符（获得value）
            defaultValue = content.substring(separatorIndex + defaultValueSeparator.length());
          }
          // 有默认值，优先替换，不存在则返回默认值
          if (defaultValue != null) {
            return variables.getProperty(key, defaultValue);
          }
        }
        // 未开启默认值功能，直接替换
        if (variables.containsKey(key)) {
          return variables.getProperty(key);
        }
      }
      return "${" + content + "}";
    }
  }

}
